Fuels derived from petroleum are customarily stored in very large quantities, and since they are flammable present a high hazard potential. Specifically, it is the hydrocarbon vapor-air mixtures of petroleum based fuels that are especially flammable, and which, if accidentally ignited, can produce explosions and costly fires. It is therefore manifest that apparatus for determining the presence and relative concentrations of hydrocarbon gases in a given area is a very useful item in preventing fires and explosions from that source.
It is well known that hydrocarbon vapors must be present in sufficient concentration to be flammable, and although various terms can be found in the literature to define this condition, the one which will be used here and which is commonly accepted in the industry at the present time, is "flammability index". Expressed in its simplest terms, the flammabilty index is the ratio of a measured hydrocarbon vapor concentration in air to the lowest concentration of the hydrocarbon in air that is flammable. That is, if the flammability index of a gaseous mixture is one or greater than one, the mixture is flammable. For all flammability index values less than one, the gaseous mixture is not flammable. It is clear that it would be a desiratum to determine quickly and easily not only the presence of hydrocarbon gases in a gas sample, but also whether or not these gases are in such concentration as to pose a risk of flammability, that is, whether the flammability index of the mixture is unity or greater than unity.
Although the risk of fire and explosion from the presence of concentrations of hydrocarbon vapors is a matter for great concern, it is not the only one. Even if hydrocarbon gases are present in concentrations below that necessary for flame propagation on contact with a source of ignition, they still may be present in such concentration as to make them injurious to the health or well-being of individuals in that environment. Accordingly, determination of this lower threshold limit in which human beings can remain on a continuous basis without adverse physical effect is also a very important matter. Since such toxic concentrations are well below those necessary to sustain flame propagation or burning, any apparatus which is to measure both flammability index and toxicity must have a correspondingly wide range of sensitivity to the hydrocarbon gases.
A known technique for determining the presence of combustible vapors and determining the flammability index of hydrocarbon based materials, such as, for example, gasoline, jet and diesel fuels, is frequently referred to as the catalytic oxidation principle. This technique involves burning a gas sample to be tested with the aid of a catalyst coated filament which is interconnected as one arm of a Wheatstone resistance bridge circuit. The temperature change of this filament resulting from the catalytic burning, unbalances the bridge providing an electric signal indicative of the concentration of the combustible materials (hydrocarbons) present in the sample. Although this approach has been widely used in the past, it is subject to a serious objection that it will not uniformly respond to all of the hydrocarbon constituents that may be present and, in particular, does not uniformly respond to alkanes, which are a principal constituent of the vapors of gasoline, jet and diesel fuels.
Another limitation of these catalytic devices is the necessity for providing a constant predetermined amount of oxygen in order to yield accurate results, since if either enriched or depleted oxygen atmospheres are used, erroneous readings and, in some cases, even complete failure of the instrument, can result. Still further in regard to these catalytic devices, it is necessary to transport the sample of the gas or vapor to be tested through a pipe or tubing to the sensing element which results in a certain amount of "hang-up" of the test vapor on the walls of the tube or fuel line, thereby providing an error in the determination since the material so hung-up does not take part in the catalytic burning. After a given measurement, hung-up molecules are not readily purged from the tubes, pipes or lines thereby possibly affecting accuracy of subsequent measurements.
There has also been developed a further approach for determining flammability utilizing a hydrogen flame ionization detector. It is basic to understanding of this approach to be aware that there is an orderly variation of the physical properties of the n-alkanes and, in particular, to understand that there is a substantially linear relationship of the carbon number of an alkane with the reciprocal of its lower flammability limit. The importance for present purposes of these interrelationships as they are applied to n-alkanes arises out of the fact that there is a preponderance of alkanes in petroleum-derived fuels and, of course, in their gaseous vapors.
In an article by Wilbur A. Affens and George W. McLaren entitled, "Flammability Properties of Hydrocarbon Solutions in Air", Journal of Chemical and Engineering Data, Volume 17, No. 4, 1972, the flammability properties of n-alkanes are described both as to vapor state and liquid fuel mixtures. Specifically, this article discusses combining Raoult's law and Dalton's law for a solution of a mixture of liquid hydrocarbons and Le Chatelier's rule for the flammability limits of vapor mixtures, for the purpose of attempting to predict the flammability index of a vapor mixture for arbitrary amounts of individual hydrocarbons.
In a further article by Wilbur A. Affens, Homer W. Carhart and George W. McLaren entitled, "Determination of Flammability Index of Hydrocarbon Fuels by Means of a Hydrogen Flame Ionization Detector", Journal of Fire and Flammability, Volume 8, 141 (April, 1977), a hydrogen flame ionization detector for measuring the flammability index of vapor-air mixtures is described. In this article, the response of the instrument is shown to be proportional to the concentration and the number of carbon atoms in the molecules of the constituent gases. Although this instrument offers some advantages over the prior referenced catalytic oxidation devices, it has some significant disadvantages, the chief one being that for proper operation it requires certain optimum flow rates of the gases for the instrument blend of the hydrocarbon and nitrogen. In addition, the described apparatus has a slow response to heavy or high carbon number hydrocarbons. It is doubtful that such a device could be made portable at a moderate cost. Still further, it is necessary to transport the sampled gas along a tube or pipeline of some kind which runs the risk of adsorbed hydrocarbon vapors or liquid fuel (hang-up), as the case may be, adding a substantial error especially when measuring relatively low concentrations of hydrocarbons for toxicity determinations.